1. Field of the Invention
The invention relates to a power storage device.
2. Description of the Related Art
A known power storage device such as a secondary battery or an electric double layer capacitor (condenser) is used as a battery for hybrid and electric vehicles. When charging and discharging, the power storage device generates heat which reduces the performance and life of the power storage device. Therefore, various technologies to cool the power storage device (or the case housing the power storage device) using coolant (gas or liquid) have been proposed.
However, as power storage devices continue to be made with greater output and higher densities, an assembled battery or battery pack formed of a plurality of single cells stacked together, for example, may not be able to be cooled sufficiently.
That is, although coolant directly contacts the outer surface of the battery pack, it does not come into contact with the region on the inside of the battery pack, which makes cooling that region difficult. More specifically, with a battery pack formed of a plurality of single cells stacked together, in which each single cell has a positive electrode body and a negative electrode body stacked together with an electrolyte layer in between, heat in the region toward the center of the battery pack cannot easily escape so the temperature in that region becomes higher than the region on the outer peripheral side of the battery pack. This results in a temperature variation between the center area and the outer peripheral area of the battery pack.
In this way, there is a variation in the temperature distribution in a plane perpendicular to the stacking direction of a single cell. If a single cell is not cooled uniformly on this plane which is perpendicular to the stacking direction, charging and discharging performance will not be stable, and moreover, it will shorten the life of the power storage device.
Therefore, related art described in Japanese Patent Application Publication No. 2004-31281 (JP-A-2004-31281) and Japanese Patent Application Publication No. 2005-71784 (JP-A-2005-71784) provides cooling fins (i.e., heat radiating fins) on a stacked battery and performs heat exchange between the inner portion of the stacked battery and coolant.
FIGS. 9A and 9B are diagrams illustrating a cooling method of related art. Typically, the power storage device is cooled by running coolant into the power storage device and discharging the coolant that has been heated by heat exchange with a plurality of power storing bodies that make up the power storage device to outside the power storage device. FIG. 9A is a diagram illustrating a cross-flow cooling method in which an inflow of coolant flowing into the power storage device which is housed in a case and an outflow of coolant that flows out from the case are in the same direction across (i.e., on opposite sides of) the power storage device (i.e., power storing bodies). FIG. 9B is a diagram illustrating a counter-flow cooling method in which the inflow and the outflow of the coolant are on the same side of the power storage device (i.e., power storing bodies) so coolant that flows into the case flows through the power storage device (i.e., power storing bodies) and then returns to the inlet side from which it flows back out of the case.
However, heat in the region toward the center of the power storage device, which is made up of a plurality of power storing bodies stacked together, has difficulty escaping, and as a result, the temperature in the region toward the center becomes higher than the temperature in the region toward the outer periphery. Therefore, the amount of heat exchange between the coolant and the power storing bodies in the region toward the center is different from the amount of heat exchange between the coolant and the power storing bodies in the region toward the outer periphery. That is, even with a structure in which the inner portion of the battery is cooled using cooling fins, as is the case with JP-A-2004-31281 and JP-A-2005-71784, it is still difficult to cool a single power storing body uniformly using the cross-flow cooling method or the counter-flow cooling method.
In other words, neither the cross-flow cooling method nor the counter-flow cooling method of the related art takes into account the variation in the temperature distribution between the region toward the center of the power storage device and the region toward the outer periphery of the power storage device. With both methods, the coolant simply flows in one direction through a plurality of flow paths. As a result, the variation in the temperature in the power storage device in a plane perpendicular to the stacking direction of the power storage device is unable to be suppressed.
Also, with both the cross-flow cooling method and the counter-flow cooling method of the related art, from the standpoint of having the coolant flow through the entire power storage device, the inflow portion and the outflow portion of the coolant are positioned apart from one another. Because a duct or the like is required for each portion, this increases the number of parts. Specifically, at least two ducts must be provided, i.e., one for the coolant inflow side and one for the coolant outflow side, and these ducts take up space. This makes it difficult to make the power storage device smaller so that it takes up less space.